NO SPOILERS. Because it is actually possible to write reviews without them. A review is not a synopsis.
You know those films where the alien invasion/contact is, after initial setbacks, defeated/handled by some super-competent shadowy government agency that had been set up years ago to deal with just this kind of science fiction plot?
This isn't one of those films.
This isn't even one of those films where, after being beat to hell and back, the plucky humans defeat the alien menace with computer viruses, or real viruses, or just by opening an alien-sized can of whoop-ass.
In this film, the "this changes everything" alien contact happens, and the governments of the world haven't the first clue about how to deal with it, and spend 20 years bickering over what course of action to take. Remind you of anything?
In this film, when a private corporation gains a limited contract to "house" the aliens (the government can't do anything, see above) it acts not in a competent and clever or even Machiavellian way, but it acts in exactly the same way that every organization that you, and I, and all of our friends, have ever worked for acts. Its promotion path is unalloyed nepotism. It knowingly pursues a path of short-term gain at the cost of long-term pain. The Peter Principle is in full effect. Remind you of anything?
And the net result of these things is, simply, a science-fiction film like no other you've ever seen.
Jaw-dropping CGI, roller-coaster action sequences, all wrapped around a big, intelligent brain and a beautifully ambiguous ending.
Sure, it's true that some of these ideas have been explored in part before. And it's also true that some of the secondary characters in the film are poorly fleshed out. It's not a perfect movie. What movie is? But it is a movie that bucks conventional sci-fi tropes, and proves once-and-for-all that you can have "future technology" actually make sense and behave in internally consistent ways without alienating (pun intended) an audience.
Finally, to derive an analogy from perhaps more well-known sci-fi, the most amazing technology in Star Trek is not the warp drive. Or the transporter. Or the phasers. The most impossible technology in Star Trek is how thousands of people manage to work really closely together on relatively tiny star ships without strict military discipline, or regularly going mad, or degenerating into destructive personal politics. Because, frankly, that very human technology is the "technology" that we're furtherest away from here in good old 2009.
District 9 reflects that sad truth like no other film before it. Four and a half stars.